1. Field of the Invention
This invention relates to a method for the production of a gas-pervious composite film and more particularly to a method for the production of a gas-pervious composite film soft to the touch and excellent in mechanical strengths.
2. Description of the Prior Art
The gas-pervious films in popular use to date have been manufactured by a process which comprises mixing a polyolefin resin with an inorganic filler, melt molding the resultant mixture in the shape of a film, an uniaxially or biaxially drawing the film thereby imparting a porous texture to the film. In the gas-pervious films of this kind, the drawing imparts to the web of film fine fractures originating in particles of the inorganic filler and enables the produced film to contain therein pores having diameters approximately in the range of 1 to 4 .mu.m. These gas-pervious films have a gas permeability of about 4,500 g/m.sup.2 .multidot.day at most. The uniaxially drawn films, however, are notably deficient in tear strength in the direction of elongation and in tensile strength in the lateral direction because of the anisotropy due to molecular orientation. The biaxially drawn films have a low expansion ratio and poor gas permeability. Moreover, owing to the rigidity inherent in any drawn film, the drawn films mentioned above possess the feeling of crispiness like paper and do not suit applications which necessitate softness.
A method has been proposed for producing a soft porous film by using a polyolefin film incorporating therein a low-melting polymer of a like species, a rubbery polymer, or an olefinic thermoplastic elastomer in addition to an inorganic filler. The film obtained by this method, however, is not sufficiently pervious to gas and is deficient in strength.
Various methods have been proposed which are aimed at imparting softness to gas-pervious drawn films composed of polyolefin resins and inorganic fillers. For example, Japanese Patent Application Disclosure No. SHO 60(1985)-257,221 discloses a method for producing a porous film excellent in softness by preparing a composition consisting of 100 parts by weight of a polyolefin resin, 25 to 400 parts by weight of a filler, and 1 to 100 parts by weight of a liquid or waxy hydrocarbon polymer or a mixture of the aforementioned hydrocarbon polymer with an epoxy group-containing organic compound, melting and extrusion molding the composition in the shape of a film, and biaxially drawing the film.
Japanese Patent Application disclosure No. SHO 62(1987)-10,141 discloses a method for the production of a porous film or sheet, characterized by preparing a composition consisting of a polyolefin resin, a filler, and triglyceride, melt modeling the composition in the shape of a film or sheet, and drawing the film or sheet. Japanese Patent Application Disclosure No. SHO 62(1987)-27,438 discloses a method for producing a gas-pervious film by at least uniaxially drawing a film composed of 42 to 87% by volume of a polyolefinic resin and 58 to 13% by volume of an inorganic filler, which method is characterized by the fact that a mixture consisting of 50 to 95% by weight of a linear low-density polyethylene and 50 to 5% by weight of a branched low-density polyethylene is used as the polyolefinic resin mentioned above and the aforementioned composition incorporates therein 3 to 25 parts by weight, based on 100 parts by weight of the composition, of an aliphatic alcohol type fatty acid ester, a compound formed between a fatty acid having 10 to 22 carbon atoms and an aliphatic alcohol having 1 to 12 carbon atoms.
Since the conventional gas-pervious films mentioned above are invariably obtained by drawing, they are thin and deficient in mechanical strengths and, therefore, must be used more often than not as superposed on some other materials. Thus, they have a disadvantage that they necessitate a secondary fabrication and entail an increase of price. Further, since they don't contain a rubbery polymer, they are deficient in flexibility and fail to acquire desired softness to the touch.
An object of this invention, therefore, is to provide a gas-pervious composite film possessing highly desirable softness to the touch and enjoying high mechanical strengths.